Two of Edward II's cousins died at the little-known battle of Vega de Granada on 25 June 1319, one of the many battles in the centuries-long Reconquista of Andalusia. Here's some information about it and about the situation in southern Spain around the time of the battle.
Most of al-Andalus, the Muslim-ruled area of the Iberian peninsula, had been re-conquered by the Christian kingdoms well before Edward II's time; Edward's grandfather Fernando III of Castile and Leon swept through it in the 1230s and 1240s, recapturing Cordoba, Seville, Jaen and numerous other towns, and Jaime I of Aragon and Afonso III of Portugal also gained lands formerly held by the Almohad rulers. The only area remaining under Muslim control in the early fourteenth century, and until it too fell to los Reyes Católicos Ferdinand and Isabella in 1492, was the sultanate of Granada, a taifa or tributary state of the Crown of Castile. From the early 1200s, Granada was ruled by the Nasrid dynasty, who rose to power after the massive defeat of their predecessors the Almohads at the battle of Las Navas de Tolosa in 1212. The sultan of Granada from 1314 was Ismail I, who deposed his uncle Nasr that year and in 1315 unsuccessfully tried to recapture Gibraltar, which had fallen to Fernando IV of Castile in 1309.
Following the long and successful reigns of Edward II's grandfather Fernando III (died 1252) and uncle Alfonso X (died 1284), the kingdom of Castile remained in turmoil for many decades. Alfonso's second son Sancho IV took the throne on Alfonso's death, ignoring the superior rights of his (Sancho's) two nephews the de la Cerda brothers, sons of Alfonso's dead eldest son Fernando de la Cerda: an act for which Alfonso cursed Sancho on his deathbed. The sudden death of Sancho IV himself in 1295 in his late thirties, leaving a nine-year-old son Fernando IV as his heir, opened Castile up to invasion by its opportunistic neighbours Aragon and Portugal, and Sancho's brother Infante don Juan claimed the Castilian throne in place of his nephew Fernando on the grounds that Sancho's marriage to his queen Maria de Molina had been invalid. The heroic efforts of Queen Maria, one of the great women of the age, saved her son's birthright, but when Fernando IV himself died suddenly in September 1312, not yet twenty-seven, he left as his heir a son, Alfonso XI, who was only eleven months old. Yet more crises and threats of invasion loomed as the kingdom faced a very long minority.
Internally, Castile was riven by conflict. The three most powerful people in the country battling for control of the baby king's realm were the dowager queen Maria de Molina, Alfonso XI's grandmother (Alfonso's mother Constanca of Portugal died in November 1313); Infante don Juan, born in 1262 or 1264, lord of Biscay, son of Alfonso X, brother of Sancho IV and Alfonso XI's great-uncle (who had claimed the throne in 1295); and Infante don Pedro, born in 1290, son of Sancho IV, brother of Fernando IV and the young king's uncle. (There were many others.) In 1315, the two infantes finally agreed to share the regency with Queen Maria, with Pedro taking control in the south and Juan in the north. The squabbling over power left Castile vulnerable not only to invasion by their Christian neighbours, but to possible renewed attacks by the Nasrids of Granada.
In 1316 and 1317, however, came temporary success for Castile: Pedro led an army to Granada and won a victory over a Nasrid army, seized a castle (though failed to capture two others), and repelled another siege of Gibraltar. In his absence, his uncle Juan tried to have himself named as sole regent of Castile, so Pedro hastily returned to deal with this situation. In early 1319, the two infantes managed to work together for once and raised an army to go against the Nasrids. On the day after the Nativity of St John the Baptist, 25 June 1319, near the hill of Sierra de Elvira, a Nasrid force surrounded and attacked the army. In appalling heat, the thirsty and exhausted Castilian army, which the two infantes had allowed to become separated, put up little resistance. Pedro was thrown from his horse and killed, either when leading a charge or as a result of fighting with some of his nobles over battle tactics. Juan fled the battlefield but died soon afterwards, presumably of his wounds, though a chronicle says he died of sorrow. Supposedly 50,000 men fell at the battle, though as always it's not recommended to give much credence to medieval chroniclers' figures, and it was claimed that the captured body of Infante don Pedro was skinned and stuffed. The force thus routed, the countryside was widely plundered in the aftermath of the battle, and the southern frontier of Castile was left open to further attack.
The most powerful regents of Castile in the aftermath of the battle were Queen Maria, until her death in July 1321; Edward II's first cousin don Juan Manuel, duke of Penafiel, a grandson of Fernando III and one of the most famous Spanish writers of the Middle Ages; Queen Maria's son don Felipe, another brother of Fernando IV and uncle of Alfonso XI; Maria Diaz de Haro, lady of Biscay, widow of don Juan who fell at Vega de Granada, and her son Juan el Tuerto, 'the one-eyed'. Two of Edward II's children were betrothed into Castile in 1324/25: Edward's elder daughter Eleanor of Woodstock (born 1318) would marry Alfonso XI, and his elder son Edward of Windsor, the future Edward III, would marry Alfonso's sister Leonor, who was some years his senior. The letters of the half-Castilian Edward II to the regents of Castile demonstrate his enthusiasm at the prospect of his children marrying into Castile, though because of his deposition, the marriages did not go ahead. Alfonso XI instead married Maria of Portugal, his first cousin on both sides (his father and her mother were brother and sister; his mother and her father were brother and sister), and they were the parents of Pedro the Cruel, king of Castile. (Maria had formerly been offered to Edward of Windsor as a bride by her father Afonso IV of Portugal, which offer Edward II politely rejected.) Alfonso XI became the only European monarch to die of the Black Death, in 1350. His son Pedro was killed in 1369 by his half-brother Enrique of Trastamara, one of Alfonso's sons with his mistress Leonor de Guzman, who made himself king of Castile. Pedro's daughters Constanza and Isabel married Edward II's grandsons John of Gaunt, duke of Lancaster, and Edmund of Langley, duke of York. Pedro himself had been betrothed to John and Edmund's sister Joan, but she died of plague in 1348 on her way to marry him. It's all rather confusing. :-)